1. Field of the Invention
The present invention relates to the bias adjustment of a radio frequency unit in a radar apparatus, and more specifically to a method for adjusting, at low cost and with good accuracy, the bias of a monolithic microwave integrated circuit (MMIC) built into a radio frequency circuit in a millimeter wave radar apparatus.
2. Description of the Related Art
It is known in the art to provide for use in a vehicle control system such as adaptive cruise control (ACC), a pre-crash system (PCS), or the like, a millimeter wave radar apparatus that uses millimeter waves in the frequency band of 30 GHz to 300 GHz as an apparatus for detecting obstacles (such as other vehicles, roadside constructions, pedestrians, etc.) around the vehicle. The currently used millimeter wave radar is of the type that employs a frequency-modulated continuous wave (FM-CW) system, and can measure the distance and relative velocity of a target by using a simple system.
A conventional millimeter wave radar apparatus comprises a transmitting/receiving antenna, a radio frequency unit connected to the transmitting/receiving antenna to transmit and receive radio waves (millimeter waves), an analog circuit incorporating a signal processing circuit for processing signals output from the radio frequency unit, a digital signal processor for digitally processing signals output from the analog circuit, and a communication interface for transmitting the signals processed by the digital signal processor to a vehicle system (ACC, PCS, etc.).
The antenna and the radio frequency unit are scanned together left and right by a motor contained in a scanner unit. The motor is driven by a motor driving circuit, which operates under the control of a signal supplied from the digital signal processor. The digital signal processor converts the beat signal produced by interference between the received wave and the transmitted wave into a digital signal, analyzes the beat frequency in its processing circuit, and detects the target's position by computing distance, relative velocity, and angle information. The radio frequency unit forming one component element of such a millimeter wave radar apparatus generally comprises a radio frequency circuit constructed from a plurality of MMICs (Monolithic Microwave Integrated Circuits) and a bias circuit for operating the MMICs.
Each MMIC contains a mixer, multiplier, amplifier, switch, etc., but the optimum point of the gate bias value for the mixer, multiplier, amplifier, etc varies in each MMIC. Further, in the radio frequency unit, since the transmit power and receive gain of the transmitter/receiver need to be adjusted to given target values, the bias value must be adjusted for each individual MMIC. Furthermore, since there exist variations in the gold ribbon connecting the MMICs, in the waveguide converter, and in the IF (Intermediate Frequency) circuit connected to the radio frequency circuit, beside variations among MMICS, bias adjustment has therefore been required.
The radio frequency unit in the millimeter wave radar apparatus includes a radio frequency circuit incorporating a plurality of MMICs, a bias circuit, and an IF circuit. Power is supplied to the radio frequency circuit and the bias circuit from a power supply circuit through respective regulators.
The MMICs built into the radio frequency circuit function as an oscillator, mixer, multiplier, amplifier, switch, etc., as described above, and are interconnected by a gold ribbon. The transmit signal output from the MMIC at the final stage is fed through the waveguide converter and the waveguide and transmitted out from the antenna. The signal reflected by the target and received by the antenna is input via the waveguide and the waveguide converter into the MMICs for processing, and the received signal processed by the MMICs is supplied to the IF circuit and sent to the analog circuit.
The analog circuit includes, in addition to the signal processing circuit, a microcomputer comprising a control processor, a memory, an I/O (Input/Output), and an external I/O. The signal from the I/F circuit is supplied via the signal processing circuit and the I/O to the control processor for processing, and the processed signal is output via the external I/O to the vehicle system.
The input side of each MMIC in the radio frequency unit is connected to a voltage dividing circuit comprising a trimmable resistor and a conventional resistor in the bias circuit. The bias circuit divides the output voltage of the regulator through the trimmable resistor and the conventional resistor, and applies the resulting bias value to the MMIC. In the prior art, the resistance set value of each trimmable resistor has been determined by examining the IF signal output from the IF circuit, and then, trimming of the trimmable resistor has been performed using the thus set value.
However, once the trimming is done, the value of the trimmable resistor can no longer be changed and as a result, there has been a problem that even if a temperature monitor is added, temperature compensation cannot be performed, and since determining whether the value is set correctly or not can only be checked in the inspection step, and therefore it is not possible to detect faults once the radar apparatus is shipped to the market.
In view of this, JP-A-2005-227031 discloses a bias adjustment method that uses a current monitor circuit for monitoring the sum of the drain currents flowing in the respective MMICs in accordance with the variations between the MMICs, and that converts the detection signal of the current monitor circuit into a digital signal, computes a gate voltage set value for producing a predetermined drain current, and applies the thus computed value to each MMIC through a D/A converter. According to the technique disclosed in JP-A-2005-227031, since expensive trimmer resistors are not used, not only the cost, but also the number of adjusting steps can be reduced compared with the conventional gate bias circuit that performs the adjustment using the bias control circuit constructed with trimmer resistors.
The technique disclosed in JP-A-2005-227031 obtains the gate voltage set value by monitoring the sum of the drain currents flowing in the respective MMICs and by determining the gate voltage set value so that the drain current value becomes equal to the predetermined current value; however, the purpose of the gate bias adjustment of the radio frequency unit is not only to adjust the optimum bias point of each MMIC, but also to adjust the radio frequency characteristics such as the transmitter power and receiver gain, and with the technique disclosed in JP-A-2005-227031 there has been the problem that highly accurate adjustment of the radio frequency characteristics cannot be accomplished because of large variations in the correlation between the drain current of each MMIC and the radio frequency characteristics.